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             Information from the European Southern Observatory

ESO Press Release 14/03

11 June 2003                                               [ESO Logo]

For immediate release
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Flattest Star Ever Seen

Part 1 of 2

VLT Interferometer Measurements of Achernar Challenge Stellar Theory

Summary

To a first approximation, planets and stars are round. Think of the
Earth we live on. Think of the Sun, the nearest star, and how it
looks in the sky.
 
But if you think more about it, you realize that this is not
completely true. Due to its daily rotation, the solid Earth is
slightly flattened ("oblate") - its equatorial radius is some 21 km
(0.3%) larger than the polar one. Stars are enormous gaseous spheres
and some of them are known to rotate quite fast, much faster than the
Earth. This would obviously cause such stars to become flattened. But
how flat?

Recent observations with the VLT Interferometer (VLTI) at the ESO
Paranal Observatory have allowed a group of astronomers [1] to obtain
by far the most detailed view of the general shape of a fast-spinning
hot star, Achernar (Alpha Eridani), the brightest in the southern
constellation Eridanus (The River).

They find that Achernar is much flatter than expected - its
equatorial radius is more than 50% larger than the polar one! In
other words, this star is shaped very much like the well-known
spinning-top toy, so popular among young children.

The high degree of flattening measured for Achernar - a first in
observational astrophysics - now poses an unprecedented challenge for
theoretical astrophysics. The effect cannot be reproduced by common
models of stellar interiors unless certain phenomena are
incorporated, e.g.  meridional circulation on the surface
("north-south streams") and non-uniform rotation at different depths
inside the star. 

As this example shows, interferometric techniques will ultimately
provide very detailed information about the shapes, surface
conditions and interior structure of stars.

The full text of this Press Release, with three photos (ESO PR Photos
15a-c/03) and all related links, is available at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-14-03.html

VLTI observations of Achernar

Test observations with the VLT Interferometer (VLTI) at the Paranal
Observatory proceed well [2], and the astronomers have now begun to
exploit many of these first measurements for scientific purposes.

One spectacular result, just announced, is based on a series of
observations of the bright, southern star Achernar (Alpha Eridani;
the name is derived from "Al Ahir al Nahr" =3D "The End of the
River"), carried out between September 11 and November 12, 2002. The
two 40-cm siderostat test telescopes that served to obtain "First
Light" with the VLT Interferometer in March 2001 were also used for
these observations. They were placed at selected positions on the VLT
Observing Platform at the top of Paranal to provide a "cross-shaped"
configuration with two "baselines" of 66 m and 140 m, respectively,
at 90=B0 angle, cf. PR Photo 15a/03.

At regular time intervals, the two small telescopes were pointed
towards Achernar and the two light beams were directed to a common
focus in the VINCI test instrument in the centrally located VLT
Interferometric Laboratory. Due to the Earth's rotation during the
observations, it was possible to measure the angular size of the star
(as seen in the sky) in different directions.

Achernar's profile

A first attempt to measure the geometrical deformation of a rapidly
rotating star was carried out in 1974 with the Narrabri Intensity
Interferometer (Australia) on the bright star Altair by British
astronomer Hanbury Brown.  However, because of technical limitations,
those observations were unable to decide between different models for
this star. More recently, Gerard T. Van Belle and collaborators
observed Altair with the Palomar Testbed Interferometer (PTI),
measuring its apparent axial ratio as 1.140 +- 0.029 and placing some
constraints upon the relationship between rotation velocity and
stellar inclination.

Achernar is a star of the hot B-type, with a mass of 6 times that of
the Sun. The surface temperature is about 20,000 degC and it is
located at a distance of 145 light-years.

The apparent profile of Achernar (PR Photo 15b/03), based on about
20,000 VLTI interferograms (in the K-band at wavelength 2.2 micron)
with a total integration time of over 20 hours, indicates a
surprisingly high axial ratio of 1.56 +- 0.05 [3]. This is obviously
a result of Achernar's rapid rotation.

Theoretical implications of the VLTI observations

The angular size of Achernar's elliptical profile as indicated in PR
Photo 15b/03 is 0.00253 +- 0.00006 arcsec (major axis) and 0.00162 +-
0.00001 arcsec (minor axis) [4], respectively. At the indicated
distance, the corresponding stellar radii are equal to 12.0 +- 0.4
and 7.7 +- 0.2 solar radii, or 8.4 and 5.4 million km, respectively.
The first value is a measure of the star's equatorial radius. The
second is an upper value for the polar radius - depending on the
inclination of the star's polar axis to the line-of-sight, it may
well be even smaller.

The indicated ratio between the equatorial and polar radii of
Achernar constitutes an unprecedented challenge for theoretical
astrophysics, in particular concerning mass loss from the surface
enhanced by the rapid rotation (the centrifugal effect) and also the
distribution of internal angular momentum (the rotation velocity at
different depths). 

The astronomers conclude that Achernar must either rotate faster (and
hence, closer to the "critical" (break-up) velocity of about 300
km/sec) than what the spectral observations show (about 225 km/sec
from the widening of the spectral lines) or it must violate the
rigid-body rotation.

The observed flattening cannot be reproduced by the "Roche-model"
that implies solid-body rotation and mass concentration at the center
of the star. The failure of that model is even more evident if the
so-called "gravity darkening" effect is taken into account - this is
a non-uniform temperature distribution on the surface which is
certainly present on Achernar under such a strong geometrical
deformation. 

 - Continued -

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